Summary of Neutralization Reactions and Their Applications

## Introduction Neutralisation is an important idea in everyday life. Farmers, gardeners and cooks use it when they balance acids and alkalis to make soils, foods and materials safe and useful. In this guide we will learn what happens when acids and alkalis meet, how to tell if a solution is acidic, neutral or alkaline, and some real-life examples you can relate to. ### What is acidity and alkalinity? - Substances have a pH value that tells us how acidic or alkaline they are. - pH ranges from 0 to 14. > **Definition:** pH is a number that shows how acidic or alkaline a solution is. Lower pH means more acid, pH 7 means neutral, higher pH means more alkali. ### pH levels and meanings | pH value | What it means | | --- | --- | | $0$ to $6.9$ | Acidic | | $7$ | Neutral | | $7.1$ to $14$ | Alkaline (basic) | Did you know that many plants grow best in soils that are slightly acidic or slightly alkaline depending on the plant? ### Testing soil pH — why farmers care - Farmers and gardeners test soil pH to know which crops will grow best. - If soil is too acidic, they add lime (an alkali) to make it more neutral. - If soil is too alkaline, they might add sulphur or other treatments to lower pH. > **Definition:** Lime is an alkali used by farmers to reduce soil acidity and help plants grow better. Table: Best pH ranges for some common crops | Plant | Best pH range | | --- | --- | | Cabbage | $5.6$–$6.6$ | | Cauliflower | $6.0$–$7.0$ | | Onions, leeks | $6.2$–$6.8$ | | Mushrooms | $7.0$–$8.0$ | | Potatoes | $5.8$–$6.5$ | Practical example: If a soil has pH $5.0$, potatoes (best $5.8$–$6.5$) will grow poorly until lime raises the pH nearer to their preferred range. ### How neutralisation works (everyday view) - When an acid and an alkali are mixed, they react and make a salt and water. - This reaction is called neutralisation because the acid and alkali cancel each other out. > **Definition:** A salt is a new substance formed when an acid reacts with an alkali. Real-life example: Hydrochloric acid in a lab reacts with sodium hydroxide (an alkali) to make table salt and water. - Word equation (simple): - acid + alkali -> salt + water Chemical example using chemical formula style: - Hydrochloric acid and sodium hydroxide form sodium chloride and water: $$\ce{HCl + NaOH -> NaCl + H2O}$$ Fun fact: Sodium chloride is the same chemical as table salt used on our food, but many other salts are made the same way, like calcium sulfate or potassium nitrate. ### How to tell when a solution is neutral (practical test) - Use universal indicator or pH paper to watch the colour and read the pH number. - A pH of $7$ means the solution is neutral. - When adding alkali to an acid, add it slowly and watch the indicator colour change to the neutral colour. > **Definition:** Universal indicator is a mixture of dyes that changes colour at different pH values and helps show whether a solution is acid, neutral or alkaline. Practical lab tip: 1. Start with the acid in a flask and measure its pH. 2. Add small amounts of alkali while stirring. 3. Watch the indicator colour change and stop when the colour shows pH $7$. 4. If you add too much alkali, the solution becomes alkaline. ### Making salts — examples - Hydrochloric acid + sodium hydroxide -> sodium chloride + water $$\ce{HCl + NaOH -> NaCl + H2O}$$ - Sulfuric acid + sodium hydroxide -> sodium sulfate + water $$\ce{H2SO4 + 2NaOH -> Na2SO4 + 2H2O}$$ - Nitric acid + ammonium hydroxide -> ammonium nitrate + water $$\ce{HNO3 + NH4OH -> NH4NO3 + H2O}$$ Did you know that some fertilisers, such as ammonium nitrate, are made by neutralising nitric acid with ammonium hydroxide? ### Safety and cleaning up spills - Small spills of concentrated acid can be made safe by carefully adding an alkali (like sodium bicarbonate) until the pH is near neutral, then mop up. - Always wear gloves and goggles and follow adult or teacher instructions. > **Definition:** Sodium bicarbonate (baking soda) is a